Standard heat treat furnaces operated at atmospheric pressure or slightly above, whether of the batch or continous type, require the use of a protective, gas atmosphere when the workpiece is subjected to a heat treatment process within the furnace. Generally, an endothermic product gas or a reducing atmosphere is required for neutral hardening, annealing or carburizing. In some heat treatment processes, such as bright hardening, brazing, carbon restoration in forgings and bar stock, sintering powder metal preforms, bright annealing, and the clean neutral hardening of all grades of steel, a reducing atmosphere generated by an endothermic gas generator is sufficient to permit the desired heat treat process to be completed. In other heat treat processes, particularly those related to case hardening such as carburizing, carbonitriding, carbohydrate nitriding, cyaniding, etc., the reducing gas atmosphere functions as a carrier gas to which an enriching gas is added (i.e. methane for carburizing) to provide an atmosphere having sufficient potential to infuse the necessary hardening elements (i.e. carbon) into the case of the workpiece. In such processes, the carrier gas is a relatively stable gas allowing the introduction of the unstable hydrocarbon to maintain a slightly pressurized furnace atmosphere without deposition of soot. The carrier gas or endothermic-base atmosphere is generally defined by the American Society for Metals as one of a series of atmospheres designated as Class 300 carrier gas typically having approximate compositions of 40% nitrogen, 40% hydrogen, and 20% carbon monoxide. A typical analysis would be as follows:
40.4% H.sub.2 PA1 19.8% CO PA1 39.0% N.sub.2 PA1 0.5% CH.sub.4 PA1 0.2% H.sub.2 O PA1 0.1% CO.sub.2
An endothermic gas produced by the reaction of natural gas and air defined by the ASM as a Type 302 carrier gas has a composition by percent and volume of 39.8% nitrogen, 20.7% carbon monoxide, 38.7% hydrogen and 0.8% methane.
There are two widely-used commercial processes for generating an endothermic carrier gas. The first process, the nitrogen methanol process, mixes methanol with nitrogen in a vaporizer and the gas mixture is then heated in a catalyst-filled retort where the methanol reacts to yield hydrogen and carbon monoxide. Alternative variations of this process simply mix nitrogen and liquid methanol from storage facilities which react within the furnace to produce the nitrogen-based endothermic gas. While there are some safety considerations which favor the nitrogen-methanol process, the cost when compared with the other commercial process is high and the type of gas produced tends to be unsuitable as a reducing atmosphere for low temperature processes such as bright annealing and spherodizing. Thus, the more widely-used commercial process uses a gas generator to mix air with methane (natural gas) in the presence of a catalyst to generate an endothermic gas of a defined composition.
The typical gas generator, is a rather large arrangement which is located outside the furnace and which is used as a sole source of an endothermic gas which is piped to any number of standard furnaces, continuous or batch, located within the heat treater's facility. More specifically, the typical gas generator comprises a retort, filled with nickel based catalyst lumps which are disposed beneath a small bed of inert lumps of a heat transfer particulate such as Al.sub.2 O.sub.3. Surrounding the retort is a source of heat. Natural gas and air are piped into the retort and externally heated to temperatures of approximately 1900.degree. to 2200.degree. F. to produce the product or carrier gas. The product gas must then be rapidly cooled below 900.degree. F. before it is piped to the heat treat furnaces to avoid reversal of the reaction and formation of soot or carbon deposition (at temperatures of 1300.degree. to 900.degree. F.) in the gas carrying ductwork. The reheating of the gas within the furnace obviously requires more energy than that which would otherwise be required.
To overcome this inherent problem, and also to overcome several other disadvantages of the typical gas generator system, an in-situ or internal gas generator has recently been developed in Japan. From available publications, it appears that this gas generator is, of necessity, a small-sized unit so that it can be applied on a retrofit basis to the heat treatment chambers of existing furnaces without interfering with the work capacity of the furnace and yet generate a sufficient volume of atmosphere to effect the heat treatment process. The invention described herein is an improvement to such internal gas generators.
In considering the development of any internal gas generator where size of the unit must be minimized, the selection and design of the catalyst arrangement within the unit is critical. In this connection, it is known from the experiences gained in the manufacture of standard gas generators, that the reaction of air and natural gas proceeds in a two-step fashion with the initial reaction being slightly exothermic and the remainder of the reaction being endothermic. Thus, the selection, sizing and positioning of the catalyst within the retort must consider the impact of the two stage reaction. With respect to endothermic reactions, U.S. Pat. No. 2,423,907 discloses the mixing of various amounts of catalyst material with inert heat transfer material to provide the highest heat output at the beginning of the endothermic reaction and the highest catalyst concentration at the end of the reaction. It is also known by U.S. Pat. Nos. 3,796,655; 2,283,499 and 3,635,943 to use differently sized catalyst particles and non-catalytic heat transfer particles. It is also known to increase catalyst activity by increasing the amount of the catalyst particles within the bed such as shown in U.S. Pat. Nos. 2,142,835 and 2,256,622. Further, varying layers of heat transfer particles with layers of catalytic material is shown in U.S. Pat. Nos. 2,248,734 and 2,423,835. Where heat must be supplied for the reaction to proceed, the heat has been furnished directly to the beds by preheated, regenerating gases.